1. Field of the Invention
The present invention relates to an adaptive Y/C separation circuit for video signal processing, in particular to a circuit that is able to automatically separate the luminance (Y) and chrominance (C) signals from the color video signals on referencing scanning lines, such that the real luminance and chrominance signals can be separated using relatively simple circuit implementation.
2. Description of Related Arts
In general, the luminance (Y) signals and the chrominance (C) signals overlap each other in composite color video signals. FIG. 7 shows a frequency spectrum of a composite color television signal with the chrominance signal (Y) completely overlapped by the luminance signal (C). The conventional means of high pass filter (HPF), low pass filter (LPF) and band pass filter (BPF) to separate the Y signal and the C signal cannot extract these two signals in whole, whilst some luminance signal component still remains in the filtered chrominance signal, or some chrominance signal component remains in the filtered luminance signal, but the filtered luminance signal lacks certain components to correspond to the chrominance signal. This discrepancy in Y/C separation often leads to blurring of the output picture and color distortion, resulting in degradation of the picture resolution.
One of the solutions often used to solve the above Y/C separation discrepancy problem is a two-dimensional separation circuit, where the Y/C signals on adjacent scanning lines, that is current scanning line, the one immediately preceding and the one immediately following (Vn+1, Vn, Vn−1), are sampled from the color video signals through a two-stage delay circuit (D1, D2), and then signals on either two out of three scanning lines that demonstrate closer values are picked out and subtracted from each other to extract the chrominance signal component (C). Since the chrominance signals on two adjacent lines (Y+C, Y−C) are 180 degrees out of phase, the luminance signal component (Y) is taken out after the subtraction, leaving the chrominance signal (C). The C signal is further subtracted from the original signal (Y+C) to obtain the luminance signal (Y). Using this technique, the color discrepancy can thus be effectively controlled.
However, the Y signals used for computation of the chrominance signal are not equal on adjacent scanning lines, so there is a small amount of the Y signal component remaining in the resultant C signal. The color discrepancy is conspicuous when the luminance output experiences large variation. Then the idea of vertical correlation to control the mixing ratio of signals on adjacent scanning lines was introduced. In FIG. 10, the signals on three adjacent scanning lines are produced through the cascaded delay circuit (D1, D2). The main difference with the previous example is that a vertical correlation circuit (80) is introduced, as shown by the dotted line portion in the diagram. Through the correlation analyzer (81) for computing the vertical correlation of chrominance signals on three adjacent scanning lines, the correlation coefficients are input to two mixers (73, 74) for ratio mixing. Through two comb filters (71, 72) the filtered luminance signal can be used for adjustment of the mixing ratio in accordance with the vertical correlation, such that the chrominance signal is able to come close to the actual chrominance level, thus improving the picture resolution.
However, since the vertical correlation circuit (80) only employs unidirectional vector computation to find the variance between the luminance signals on adjacent scanning lines, the resultant value is still unable to meet the low discrepancy criterion. One way to solve the problem is to increase the number of referencing scanning lines from three to five, and another way is to introduce the analysis of two adjacent frames which requires a frame buffer (F), as shown in FIG. 11. A three-dimensional separation circuit is needed for the analysis of two frames. The biggest disadvantage of a three-dimensional separation circuit is the massive amount of data that are needed for the computation of the picture frames. With such a large amount of processing data the related circuit design gets very complicated and also is not very cost effective for commercial use.